The aboveground biomass of Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) and annual production over 7 years was studied in relation to thinning and nitrogen fertilization. Biomass yield of both treatments increased during the first 3–4 years then decreased for fertilization but not with thinning. Treatments doubled biomass production of individual trees over the study period when applied separately and quadrupled it when combined. Annual biomass production per unit of foliage (E) increased during the first 3–4 years, but was at or below control level after 7 years. The increased E accounted for 20, 37, and 27% of the stemwood dry matter response to thinning, fertilization and the combined treatments, respectively; the remainder was attributed to an increase in foliage biomass. Thinning, but not fertilization, influenced distribution of radial growth along the stem, increasing growth only below the top one-third of the stem. This pattern was related to crown development.
The effect of two levels of thinning (zero and [Formula: see text] of basal area removed) and three levels of nitrogen fertilization (0,224, and 448 kg N/ha) on crown development of codominant, 24-year-old Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) trees was studied over a 5- to 7-year period. Thinning and heavy ferilization separately increased needle mass per tree after 7 years by 90% and, when combined, by 271%. Yearly needle production peaked 2–3 years after fertilization and resulted from an increase in needle size, needle number per shoot, and number of shoots produced. Maximum foliage mass per tree was reached 4–7 years after fertilization. Thinning effect on needle production was lower initially, but increased throughout the study period. A continuing foliage production in branches low in the crown contributed to thinning effect on foliage mass and crown size. Foliage distribution was affected most in the top half of the crown by fertilization and in the bottom half of the crown by thinning. Fertilization increased branch elongation at all crown heights, but thinning alone had no effect on crown width down to whorl 12.
Single tree plots in a 23-year-old stand of Douglas-fir were: (1) fertilized at a rate of 448 kg nitrogen (N) per ha with NH4NO3 in April; (2) irrigated in the summer of the same year and in the following year with 25 mm of water per week; (3) given the combined fertilizer and irrigation treatment, or (4) left untreated. Diameter growth at breast height (1.37 m) increased in the first year by 16, 15 and 59%, respectively, above that of the control. Similar effects were obtained at breast height in the second year and also at the one-half tree height position. Only earlywood production was affected significantly; this resulted from an increase in rate and length of time of earlywood production. Treatment effect on nitrogen, phosphorus (P) and potassium (K) concentration of leaves, leaf growth and water stress of soil and trees, was studied. Rate of photosynthesis was reduced similarly by water stress for fertilized and unfertilized trees; an increase in CO2 diffusion resistance appeared to cause the reduction in rate. Tree water stress had a more adverse effect on stem diameter growth than on rate of photosynthesis. The interaction between irrigation and fertilization indicates that nitrogen fertilization will have the most effect on stem diameter growth in years and on sites with favorable soil-water conditions.
A 24-year-old Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) stand was treated with various levels and combinations of nitrogen fertilization and thinning. Over a period of 5–9 years after treatments, trees were sampled to determine effects on foliage quantity and sapwood characteristics at different stem heights together with their relationships. Sapwood width remained relatively constant up the stem where heartwood was present, but the number of annual rings it contained decreased with height. The sapwood width at breast height (bh) increased with stem diameter; treatments had little effect on percent sapwood at bh. The ratio of foliage mass to sapwood cross-sectional area changed for different portions of the crown and was lower when based on sapwood area at bh than at base of live crown. Significant linear relationships of foliage mass and area to sapwood area at bh were found, but relationships of foliage to basal area (bh) were just as close for all treatments; treatments significantly affected these relationships with control trees having the lowest regression slopes.
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